Glass manufacturing process are known for producing B.sub.2 O.sub.3 and CaO containing glasses and which glasses can, optionally, include fluorine. Numerous such glasses have been produced in the past by supplying batch ingredients including calcined colemanite to a combustion-fired glass melter and melting the ingredients in the melter while withdrawing the flue gases from the furnace and passing them through a heat exchanger, e.g., a regenerator or recuperator. Such gases will contain boron values and when fluorine is present in the glass, they will likewise include fluorine values.
One such process is disclosed in U.S. Pat. No. 4,184,861 which is hereby incorporated by reference. Such patent discloses a process for producing a calcia and boric oxide containing glass, which glass can optionally include fluorine, in a combustion fired melter. The process comprises converting glass forming batch ingredients, including calcined colemanite, into agglomerates, heating a bed of such agglomerates in a chamber by direct contact with flue gases so as to produce heated, non-aggregated agglomerates which agglomerates are then supplied to the melter. The gases which are employed for the direct heating of the bed originate in the melter and have been cooled in a heat exchanger prior to such direct contact. These gases will contain boron values and, when fluorine is present in the glass being manufactured, they will likewise include fluorine values. Such gases after they have passed through the heat exchanger, e.g., a recuperator or regenerator, typically will have a temperature of about 1400.degree. F.-1500.degree. F. (about 760.degree. C.-816.degree. C.). More generally, however, they will typically have a temperature in excess of about 800.degree. F. (427.degree. C.).
As the gases pass through the bed of agglomerates, they not only heat the agglomerates, thereby salvaging some of the sensible heat in such gases, but a pollution abating feature is also realized in that some of the pollutants are separated in the bed by a complex physiochemical mechanism. Additionally it is known that raw colemanite, rather suddenly and violently, releases its chemically bound water, i.e., it decrepitates. Such descepitation generally occurs at a temperature of about 390.degree. C.-410.degree. C. This decrepitation is sufficiently violent that it can cause severe dusting problems even in a particulate batch. When agglomerates are employed, however, the decrepitation causes a more severe problem, in that it causes the agglomerates to disintegrate. In the past, colemanite was either purchased, as calcined colemanite, or prior to usage the raw colemanite was separately treated by heating it above its decrepitation temperature to release substantially all of its chemically bound water, or water of crystalization. In either case, it will be appreciated that the glass manufacturing process was detrimentally impacted upon in that the cost of raw materials is increased by the necessary procurement of such calcined colemanite.
With the foregoing in mind, the present invention has as its object to decrease the raw material costs of such a process and substantially simultaneously enhance the pollution abating characteristics by providing for the recovery of some of the boron values and, when fluorine is present in the glass, for the recovery of fluorine values. More specifically, the present invention provides for the substantially simultaneous calcination of colemanite and a reaction to minimize the amount of boron values, and when present, fluorine values, which previously would need to have been separated in the bed of agglomerates or by various types of pollution abating equipment, for example, scrubbers, cyclones or baghouses.
The foregoing is accomplished by introducing raw colemanite into the flue gases, after they have been cooled in the heat exchanger, but prior to their being employed to heat the bed of agglomerates. Since these gases will, as indicated above, have a temperature which is in excess of the decrepitation temperature of raw colemanite, a decrepitated, or calcined, colemanite will be produced. Substantially contemporaneously with the formation of such decrepitated colemanite will be a reaction between the boron values in such flue gass and, when fluorine is present in the glass, a reaction of fluorine values will also occur. The exact sequence of events is not presently known, but it will be found that such reaction does occur. More specifically, it is not known whether such reaction will occur prior to the raw colemanite having reached its decrepitation temperature or whether it will occur substantially at such decrepitation temperature or whether it will occur subsequent to the colemanite having been heated above its decrepitation temperature. Consequently, as used herein, the terminology that a boron reacted, and when present, fluorine reacted calcined, or decrepitated, colemanite is formed comprehends any of the above possible sequences. The solid, boron reacted and, when present, fluorine reacted calcined, or decrepitated, colemanite is separated from the gas stream, preferably by a cyclone, prior to then heating a bed of agglomerates of glass forming batch material with such gases. Thus, it will be seen that in this manner the raw colemanite is converted into a suitably usable calcined form and likewise, some of the pollutants are recovered and recycled as raw materials into the batch. In this way, there is a favorable impact on the economics of the glass manufacturing process. Similarly, since some of the pollutants will be recovered prior to passing of the gases through the bed, it will be appreciated that pollution abatement features will be enhanced since the bed now, instead of operating as a primary pollution abatement device, will serve more in the nature of a secondary or polishing pollution abatement device.
The terminology "boron values" and "fluorine values" comprehends within the scope of any of the compounds existing in a glass manufacturing process which contain boron or fluorine and includes elemental forms thereof. Exemplary of such compounds are H.sub.3 BO.sub.3, HBO.sub.2, HF, BF.sub.3.